Cephalaspis lyelli was a small fish, 20–30cm long with the following characteristics:

  • the head-shield is large - about a third of the length of the body
  • the shield is nearly as wide as it is long
  • the cornua are well developed and project backwards and outwards
  • the orbits are small and roughly in the middle of the shield - in the type specimen the head shield is about 8cm long, and 7.8cm wide
  • there is a small - 2cm - dorsal spine at the back of the shield, and the cornua are 2.8cm long
  • the inner margin of the cornua are smooth, and do not possess tooth-like structures called denticles, often seen in other genera
  • Cephalaspis lyelli has three openings in the skull ('fields') that are filled with small scales
  • the central dorsal field, just behind the eyes, is long, straight sided and narrow, 2.2cm long, and 0.7cm wide, at the widest point
  • the lateral fields near the edge of the shield extend onto the cornua, and are relatively broad
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